1. Field of the Invention
The present invention relates to an optical scanning apparatus.
2. Discussion of Related Art
A currently used optical scanning apparatus is arranged such that a light beam from a light source is caused to be deflected at a constant velocity, via a light deflecting system having a deflective reflecting surface. The deflected light beam, effected by the scanning image formation lens, is converged into a beam spot on a surface to be scanned, thus causing a beam spot to scan the surface to be scanned at a constant speed. In general, such an optical scanning apparatus is commonly used in an information writing device of a laser printer, a facsimile machine, or a digital copy machine.
As a light deflecting system for deflecting a light beam introduced into a scanning image formation lens, a polygon mirror has been commonly used. Because the rotating center axis of the polygon mirror is generally located at a position that is not coincident with the optical axis of the scanning image formation lens, a rotation of the polygon mirror will cause a change in a relationship between a deflective reflecting point and a scanning lens, producing a phenomenon called "sag".
When such a sag condition occurs, it is very likely that deterioration of the written image will occur, particularly in an image curvature in a sub-scanning direction (hereinafter referred to as "sub-scanning image curvature").
More specifically, in an optical scanning apparatus using a polygon mirror, in order to correct a surface tilt of the polygon mirror, an incident light beam introduced into a scanning image formation lens is not a parallel light beam with respect to the sub-scanning direction. With a change of the deflective reflecting point, an image formation position in the sub-scanning direction will be changed, resulting in deterioration of the sub-scanning image curvature.
A deflected light beam introduced into the scanning image formation lens, with respect to the main scanning direction, is allowed to be a parallel light beam or is allowed to be a light beam having a certain divergence or a certain convergence, thereby providing some technical merits when being used in various conditions. On the other hand, if a deflected light beam to be introduced into the scanning image formation lens is a light beam having a certain convergence or a certain divergence in the main scanning direction, an influence possibly brought about by the sag is likely to cause a deterioration in the image curvature in the main scanning direction (hereinafter referred to as "main scanning image curvature") and in a constant scanning speed property, as in the sub-scanning direction.
As a method for reducing an influence possibly caused due to the sag, it has been known that a scanning image formation lens may be shifted or tilted within a main scanning plane (ideally, in a plane where the main light ray of a deflected light beam is just sweeping). However, with this method, both the main scanning image curvature and the sub-scanning image curvature are difficult to be completely corrected at the same time. Therefore, this method is not sufficient for controlling a diameter change of a beam spot when such a beam spot diameter has already been made small in order to obtain a high writing density for information recording. As a result, the above method cannot satisfy the demand for recording information with a high writing density.
Further, as a method for correcting an influence which will possibly be caused by the sag, there has been suggested another method in which a sub-scanning radius of curvature of a scanning image formation lens is non-symmetrical with respect to an optical axis (Japanese Unexamined Patent Publication No. 2-23313; Japanese Examined Patent Publication No. 7-69521; Japanese Unexamined Patent Publication No. 7-113950; Japanese Unexamined Patent Publication No. 8-122635; Japanese Unexamined Patent Publication No. 8-297256).
With the use of the method as suggested in the above-noted Japanese Patent Publications, because an image formation position of a light beam is coincident with a scanned surface with respect to each image height, in principle, it has become possible to completely correct a sub-scanning image curvature irrespective of the existence of the sag.
Recently, with a remarkable development in obtaining a high density for information writing, a demand for making a diameter of a light beam spot smaller and improving stability in obtaining a light beam spot having a smaller diameter has increased. In order to satisfy such a demand, it has become important that not only the image curvature but also the optical magnification of a scanning optical system be uniformly set irrespective of what an image height is. If a beam spot has a change in its optical magnification according to an image height, the waist diameter of the beam spot will also change substantially in proportion to a lateral magnification. As a result, it is impossible to obtain "a stabilized beam spot" capable of avoiding a change in the diameter of a beam spot, which change will otherwise be caused according to an image height.
In any of the inventions described in the above-noted Japanese patent publications, a sub-scanning radius of curvature has a monotone change (having no inflection point). As a result, it is difficult to ensure a uniformity for an optical magnification while also correcting an image curvature. Further, if an optical system is constructed only of a lens surface which has a sub-scanning radius of curvature having a monotone change, the scanning image formation lens will require more than two lens elements. In addition, a change in the radius of curvature will become large between the vicinity of an optical axis and the peripheral areas, resulting in a problem that a lens will have a large difference in its thickness between the vicinity of the optical axis and the peripheral areas, and such a lens is difficult to make of a plastic material in an extrusion molding process.
In addition, another scanning image formation lens has a cross section in a sub-scanning direction in the vicinity of an optical axis having a biconvex shape and the sub-scanning radius of curvature has a plurality of extreme values (Japanese Unexamined Patent Publication No. 10-148755).
However, because such kind of a scanning image formation lens has a biconvex shape, it is impossible to freely set a principal point position for each image height, hence rendering it difficult to maintain a constant optical lateral magnification for each image height, resulting in a problem that the diameter of a beam spot changes.